226 Pla?it Biology 



V. ABSORPTION BY PLANTS 



Water. — Higher plants absorb water and inorganic salts by means of 

 the fine, transparent root hairs located near the tip of their roots (Fig. 

 57). The root hairs are really extensions of the epidermal cells and are 

 in close contact with the film of water which surrounds the individual 

 grains of the soil. Water passes from this film through the cell wall and 

 plasma membrane of the root hairs and the epidermis of the root tip. 

 Water is not absorbed by any other part of the root, stem, or leaves. The 

 absorption of water depends on the osmotic pressure of the soil water 

 and the osmotic pressure of the protoplasm within the cells. Water sup- 

 plies two elements, hydrogen (H) and oxygen (O), for plant use, in addi- 

 tion to serving as a vehicle for the entrance of essential inorganic salts. 

 Only a small part of the absorbed water remains in a plant as will be 

 seen when transpiration is considered. 



Inorganic Salts. — Soils usually contain a variety of salts, some of 

 which can be used by a plant. Certain of these salts are selected by the 

 plant and absorbed for its future use. Plants require certain essential 

 salts which contain the following chemical elements: calcium (Ca), 

 potassium (K), nitrogen (N), phosphorus (P), magnesium (Mg), sul- 

 fur (S), and iron (Fe). These are rarely found in the soil as elements 

 but are frequently combined into such usable compounds as calcium 

 nitrate, Ca(N03)2; potassium nitrate, KNO3; potassium phosphate, 

 KH2PO4; magnesium sulfate, MgS04; iron phosphate, FeP04. The 

 quantity and quality of the chemicals in the soil, together with its water 

 content, to a great degree determine the structure and functions of the 

 plant growing in that particular type of soil. 



VI. TRANSPIRATION BY PLANTS 



The roots continually absorb water which is conducted by the xylem 

 through the stems and leaves (Figs. 53 and 55 to 60). Most of this 

 absorbed water is given off by the leaves through the process of transpira- 

 tion. The epidermal cells of the leaf contain a waxy substance (cutin) 

 which prevents any great loss of moisture through those cells. Most of 

 the transpiration occurs through the minute stomata scattered through- 

 out the upper and lower epidermis, particularly the latter. The xylem 

 tissues transport the water to the cells of the mesophyll of the leaf. From 

 the spongy tissue of the mesophyll, the water escapes into the intercellular 

 air spaces, from which it evaporates through the stomata. This escape 

 of water is regulated by the pair of semicircular guard cells which sur- 



